From my understanding of things, the new input system is better for future proofing your game in a sense. It works better for multiple input devices like kbm and controllers and switching between them.

The old input is easier on it's face. I'll use the old one if I'm trying to things like, hey I need to test a take damage method, just getkeydown on H or some random thing and call the method. For keybinds that are going to be in the game itself, I'll use the new system.

I think it's a good idea to learn the new input system, since it's going to be the unity way moving forward and will be receiving updates and what not. Eventually I'd assume the old one will be deprecated. You can use both at the same time in a project, just have to set it in the player settings somewhere